Hi, if ever the 'tab button' is clicked at least once, that tab selection grid (as shown in the picture below) around controls appear to indicate which control is currently selected.

But once it is clicked, it won't disappear even if the user continues to work with the mouse only.

My Aim is:

To remove that selection grid. Where should I go (in which direction) to get a chance of achieving this please?

I included a zip of the classes I decompiled from System.windows.forms that is used by the combobox. I couldn't find a definite answer to what was responsible for drawing the focus rectangle but it appeared that it send a windows message to the system and windows draws it. I couldn't be sure without testing though.